The present invention relates to a variable displacement compressor for vehicle air-conditioning.
FIG. 8 shows a prior art variable displacement compressor. A drive shaft is rotatably supported in the housing 101, which encloses a crank chamber 102. A lip seal 104 is located between the housing 101 and the drive shaft 103 to prevent leakage of fluid from the housing 101.
An electromagnetic friction clutch 105 is located between the drive shaft 103 and the engine Eg, which serves as a power source. The clutch 105 includes a rotor 106 that is coupled to the engine Eg, an armature 107 that is fixed to the drive shaft 103, and an electromagnetic coil 108. When the coil 108 is excited, the armature 107 is attracted to and contacts the rotor 106. In this state, power of the engine Eg is transmitted to the drive shaft 103. When the coil 108 is de-excited, the armature 107 is separated from the rotor 106, which disconnects the power transmission from the engine Eg to the drive shaft 103.
A lug plate 109 is fixed to the drive shaft 103 in the crank chamber 102. A thrust bearing 122 is located between the lug plate 109 and the housing 101. A swash plate 110 is coupled to the lug plate 109 via a hinge mechanism 111. The swash plate 110 is supported by the drive shaft 103 such that the swash plate 110 slides axially and inclines with respect to the axis L of the drive shaft 103. The hinge mechanism 111 causes the swash plate 110 to integrally rotate with the drive shaft 103. When the swash plate 110 contacts the limit ring 112, the swash plate 110 is positioned at the minimum inclination position.
The housing 101 includes cylinder bores 113, a suction chamber 114, and a discharge chamber 115. A piston 116 is accommodated in each cylinder bore 113 and is coupled to the swash plate 110. A valve plate 117 partitions the cylinder bores 113 from a suction chamber 114 and a discharge chamber 115.
When the drive shaft 103 rotates, the swash plate 110 reciprocates each piston 116. Accompanying this, refrigerant gas in the suction chamber 114 flows into each cylinder bore 113 through the corresponding suction port 117a and suction valve 117b, which are formed in the valve plate 117. Refrigerant gas in each cylinder bore 113 is compressed to reach a predetermined pressure and is discharged to the discharge chamber 115 through the corresponding discharge port 117c and discharge valve 117d, which are formed in the valve plate 117.
An axial spring 118 is located between the housing 101 and the drive shaft 103. The axial spring 118 urges the drive shaft 103 frontward (leftward in FIG. 8) along the axis L and limits axial chattering of the drive shaft 103. A thrust bearing 123 is located between the axial spring 118 and an end surface of the drive shaft 103. The thrust bearing 123 prevents transmission of rotation from the drive shaft 103 to the axial spring 118.
A bleed passage 119 connects the crank chamber 102 to the suction chamber 114. A pressurizing passage 120 connects the discharge chamber 115 to the crank chamber 102. A displacement control valve, which is an electromagnetic valve, adjusts the opening size of the pressurizing passage 120.
The control valve 121 adjusts the flow rate of refrigerant gas from the discharge chamber 115 to the crank chamber 102 by varying the opening size of the pressurizing passage 120. This varies the inclination of the swash pate 110, the stroke of each piston 116, and the displacement.
When the clutch 105 is disengaged, or when the engine Eg is stopped, the control valve 121 maximizes the opening size of the pressurizing passage 120. This increases the pressure in the crank chamber 102 and minimizes the inclination of the swash plate 110. As a result, the compressor stops when the inclination of the swash plate 110 is minimized, or when the displacement is minimized. Accordingly, since the displacement is minimized, the compressor is started with a minimal torque load. This reduces torque shock when the compressor is started.
When the cooling load on a refrigeration circuit that includes the compressor is great, for example, when the temperature in a vehicle passenger compartment is much higher than a target temperature set in advance, the control valve 121 closes the pressurizing passage 120 and maximizes the displacement of the compressor.
Suppose that when the compressor is operating at maximized displacement, it is stopped by disengagement of the clutch 105 or by shutting off the engine Eg. In this case, the control valve 121 quickly maximizes the opening size of the closed pressurizing passage 120 to minimize the displacement. Also, when the vehicle is suddenly accelerated while the compressor is operating at maximum displacement, the control valve 121 quickly maximizes the opening size of the pressurizing passage 120 to minimize the displacement and to reduce the load applied to the engine Eg. Accordingly, refrigerant gas in the discharge chamber 115 is quickly supplied to the crank chamber 102. Though some refrigerant gas flows to the suction chamber 114 through the bleed passage 119, the pressure in the crank chamber 102 quickly increases.
Therefore, the swash plate 110, when at a minimum displacement position (as shown by the broken line in FIG. 8) is pressed against a limit ring 112. Also, the swash plate 110 pulls the lug plate 109 in a rearward direction (rightward in FIG. 8) through the hinge mechanism 111. As a result, the drive shaft 103 moves axially rearward against the force of the axial spring 118.
When the drive shaft 103 moves rearward, the axial position of the drive shaft 103 with respect to a lip seal 104, which is held in the housing 101, changes. Generally, a predetermined contact area of the drive shaft 103 contacts the lip seal 104. Foreign particles such as sludge exist on the peripheral surface of the drive shaft 103 that is outside the predetermined contact area. Therefore, when the axial position of the drive shaft 103 with respect to the lip seal 104 changes, the sludge will be located between the lip seal 104 and the drive shaft 102. This lowers the sealing performance of the lip seal 104 and may cause leakage of refrigerant gas from the crank chamber 102.
When the operation of the compressor is stopped by the disengagement of the clutch 105 and the drive shaft 103 moves rearward, the armature 107, which is fixed to the drive shaft 103, moves toward the rotor 106. The clearance between the rotor 106 and the armature 107 when the clutch 105 is disengaged is set to a small value, for example, 0.5 mm. Accordingly, when the drive shaft 103 moves rearward, the clearance between the rotor 106 and the armature 107 is eliminated, which causes the armature 107 to contact the rotating rotor 106. This may cause noise and vibration or may transmit power from the engine Eg to the drive shaft 103 regardless of the disengagement of the clutch 105.
When the drive shaft 103 moves rearward, each piston 116, which is coupled to the drive shaft through the lug plate 109 and the swash plate 110, also moves rearward. This moves the top dead center position of each piston 116 toward the valve plate 117 which may permit the pistons 116 to collide with the valve plate 117. Since the control valve 121 maximizes the opening size of the pressurizing passage 120 during sudden accelerations of the vehicle while the compressor is operating, the rearward movement of the drive shaft 103 accompanying the control may cause the pistons 116 to repeatedly collide with the valve plate 117. This generates noise and vibration.
To prevent the rearward movement of the drive shaft 103, the force of the axial spring 118 can be increased. However, increasing the force of the axial spring 118 lowers the durability of the thrust bearing 123, which is located between the axial spring 118 and the drive shaft 103, lowers the durability of the thrust bearing 122, which is located between the housing 101 and the lug plate 109, and increases the load placed on the engine by the compressor.